1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a color liquid crystal display device for coloring a background or display regions in a single color or a plurality of colors.
2. Description of the Related Art
Conventionally, several schemes have been proposed for a single-color liquid crystal display device for coloring a background or display regions of the liquid crystal display device.
A first conventional example is a single-color liquid crystal display device having a color polarizing film disposed outside a liquid crystal display element The device has a simple structure and is generally used in many applications.
A second conventional example is a single-color liquid crystal display device in which a dichroic dye is mixed in a nematic liquid crystal of a liquid crystal element to cause the dichroic dye to function with the operation of nematic liquid crystal molecules, which is called a guest-host scheme.
However, such conventional single-color liquid crystal display devices display characters or figures colored with a dye or a dichroic dye on a white background, or conversely, display white characters or figures on a background colored with a dye or a dichroic dye, resulting in a low contrast.
The above-described liquid crystal display devices also have a disadvantage of limited number of colors to be displayed due to a limited number of dyes or dichroic dyes available.
Thus, as a third conventional example, a single-color liquid crystal display device composed of a single polarizing film, a TN liquid crystal element having a twist angle of 90 degrees, a retardation film for circularly polarized light, a cholesteric liquid crystal film, and a light absorbing film has been proposed. The single-color liquid crystal display device displays brightly colored characters or figures on a black background utilizing a selective scattering phenomenon, or conversely, displays black characters or figures on a brightly colored background, thus obtaining display with a higher contrast.
Also, the single-color liquid crystal display device can display optional colors only by adjusting a twist pitch in the cholesteric liquid crystal film to thereby realize a display device for colorful single-color display.
Incidentally, such conventional arts are disclosed, for example, in Japanese Laid-open Patent Application No. 52-5550 and Japanese Laid-open Patent Application No. 6-230362.
However, the above-mentioned single-color liquid crystal display device using a cholesteric liquid crystal film has disadvantages in that the viewing angle characteristic is poor due to the use of a TN liquid crystal element and it is difficult to apply the device to a high density liquid crystal display device having 100 or more scanning lines.
Additionally, the above-mentioned device provides only single-color display since only one cholesteric liquid crystal film is used.
The device also has a disadvantage in that backlight illumination can not be utilized due to the use of a light absorbing film which does not transmit light, thereby reducing nighttime or low ambient light visibility.
The present invention is made to solve the above-mentioned disadvantages and a first object of the present invention is to provide a liquid crystal display device which has a good viewing angle characteristic and can perform single-color display with a colorful tone and a higher contrast even in a high-density liquid crystal display device having 100 or more scan lines.
A second object of the present invention is to provide a liquid crystal display device which can perform multi-color display in two or more colors and with a colorful tone and a higher contrast.
A third object of the present invention is to provide a liquid crystal display device which realizes sufficient brightness even at night using backlight illumination.
To achieve the above-mentioned first object, the present invention provides a liquid crystal display device composed of: an STN liquid crystal element arranged to sandwich a nematic liquid crystal twist-aligned at an angle ranging from 180 degrees to 270 degrees between a first substrate having a first electrode and a second substrate having a second electrode; a retardation film provided outside the second substrate; a polarizing film provided outside the retardation film; a retardation film for circularly polarized light provided outside the first substrate; a cholesteric liquid crystal polymer sheet provided outside the retardation film for circularly polarized light; and a light absorbing film provided outside the cholesteric liquid crystal polymer sheet.
A twisted retardation film can be used as the retardation film.
Also, the liquid crystal display device can be composed of: a TN liquid crystal element arranged to sandwich a nematic liquid crystal twist-aligned at an angle of approximately 90 degrees between a first substrate having a first electrode and a second substrate having a second electrode; a polarizing film provided outside the second substrate; a retardation film for circularly polarized light provided outside the first substrate; first cholesteric liquid crystal polymer sheets provided outside the retardation film for circularly polarized light; a second cholesteric liquid crystal polymer sheet provided outside the first cholesteric liquid crystal polymer sheet; and a light absorbing film provided outside the second cholesteric liquid crystal polymer sheet, wherein the first cholesteric liquid crystal polymer sheet and the second cholesteric liquid crystal polymer sheet have different scattering center wavelengths and have respective twist directions in the same direction.
To achieve the above-mentioned second object, the present invention provides the above-mentioned liquid crystal display device in which the cholesteric liquid crystal polymer sheet includes a plurality of cholesteric liquid crystal polymer sheets provided outside the retardation film for circularly polarized light, the light absorbing film is provided outside the outermost cholesteric liquid crystal polymer sheet within the plurality of cholesteric liquid crystal polymer sheets, and the plurality of cholesteric liquid crystal polymer sheets have scattering center wavelengths different from one another and have respective twist directions in the opposite directions.
Also, as the above-mentioned plurality of cholesteric liquid crystal polymer sheets, a first cholesteric liquid crystal polymer sheet and a second cholesteric liquid crystal polymer sheet having twist directions in the same direction can be provided and a half-wavelength retardation film can be provided between the first and second cholesteric liquid crystal polymer sheets.
A liquid crystal display device according to the present invention can be composed of: a TN liquid crystal element, a polarizing film provided outside a second substrate thereof; a reflection-type polarizing film provided outside a first substrate thereof; a retardation film for circularly polarized light provided outside the reflection-type polarizing film; a cholesteric liquid crystal polymer sheet provided outside the retardation film for circularly polarized light; and a light absorbing film provided outside the cholesteric liquid crystal polymer sheet.
In these liquid crystal display devices, a light diffusing layer or a light diffusing sheet can be provided on an outer surface of the polarizing film.
Additionally, to achieve the above-mentioned third object, the present invention uses a translucent light absorbing film as the light absorbing film and a backlight is provided outside the translucent light absorbing film.
To achieve the above-mentioned second object, a liquid crystal display device according to the present invention uses three PA liquid crystal elements, each of which is arranged to sandwich a nematic liquid crystal with zero-degree twist alignment between a first substrate having a first electrode and a second substrate having a second electrode, a retardation film for circularly polarized light is provided outside the second substrate of a first PA liquid crystal element, a polarizing film provided outside the retardation film for circularly polarized light, a first cholesteric liquid crystal polymer sheet, a second PA liquid crystal element, a second cholesteric liquid crystal polymer sheet, a third PA liquid crystal element, a third cholesteric liquid crystal polymer sheet, and a light absorbing film are sequentially disposed outside the first substrate, and the first, second, and third cholesteric liquid crystal polymer sheets have scattering center wavelengths different from one another to thereby compose a color liquid crystal display device which enables multi-color display.
Alternatively, three TN liquid crystal elements are used, a polarizing film is provided outside a second substrate of a first TN liquid crystal element, a first retardation film for circularly polarized light, a first cholesteric liquid crystal polymer sheet, a second retardation film for circularly polarized light, a second TN liquid crystal element, a third retardation film for circularly polarized light, a second cholesteric liquid crystal polymer sheet, a fourth retardation film for circularly polarized light, a third TN liquid crystal element, a fifth retardation film for circularly polarized light, a third cholesteric liquid crystal polymer sheet, and a light absorbing film are sequentially disposed outside a first substrate of the first TN liquid crystal element, and the first, second, and third cholesteric liquid crystal polymer sheets have scattering center wavelengths different from one another to thereby also compose a color liquid crystal display device which enables multi-color display.
The above and other objects, features and advantages of the invention will be apparent from the following detailed description which is to be read in conjunction with the accompanying drawings.